|Balloons produced by Interface are made using tightly controlled extrusion processes.
There’s a lot more to extruding medical device tubing than squeezing some plastic through an opening. First, it all starts with the materials: How do you choose the right materials for the right job? Second, you need to control a host of process parameters: How do you maintain proper temperatures, pressures, or drying times? Third, you need to meet tight tolerances: How do you achieve thin walls and small diameters? In short, a lot of materials science goes into the art of extrusion.
Grasping Polymer Science
“To create extruded medical device tubing, many engineers today are interested in metal-to-thermoplastic and PVC-to-thermoplastic conversion,” remarks Dwain Tarmey, project manager at Carrick-on-Shannon, Ireland–based VistaMed. “High-temperature polymers are replacing metal because they offer excellent tensile strength and a good flexural modulus. In addition, they are lighter, cheaper, more flexible, and easier to process than metals.” Thanks to ongoing improvements in the physical and mechanical properties of medical device thermoplastic polymers, new plastic options such as PEEK, polyurethanes, and polyolefins are being substituted for long-established materials.”
Take PEEK, for example. PEEK is replacing the use of stainless steel and other metals in the manufacture of medical device tubing because it is very strong and has a low coefficient of friction. Thus, it is suitable for such applications as load-bearing implants.
Or take the case of polyurethanes. Although polyurethanes are more expensive than PVC, their use continues to grow because they have a wide hardness range and a unique chemistry that renders them highly versatile. Thus, they are often used to make over-the-needle IV catheters, central venous access catheters, and multilumen catheters. “Polyurethanes are segmented polymers, meaning that they have a soft segment that provides flexibility and a hard segment that provides column strength,” Tarmey comments. “Polyurethane-based catheters, therefore, can be advanced in the body without kinking.”
Like polyurethanes, polyolefin-based materials are also replacing PVC in many applications. Because of their excellent physical properties, they can reduce material thickness in many medical device applications.
“A good understanding of polymer science and the behavior of polymer materials is extremely important for achieving high-quality extruded tubing,” Tarmey states. “In their solid state, for example, polymers can exhibit two types of morphologies: amorphous and semicrystalline. To form balloons, it is critical that the extruded tubing be in an amorphous state prior to the balloon-forming process.”
When extruding a semicrystalline polymer such as PEEK, for example, the polymer chains begin to change to an amorphous state during the die-swell stage, Tarmey explains. However, this transformation can cause compression forces to occur in the polymer melt. If these stresses are not relieved, the material will be in a highly stressed state, resulting in such problems as shrinkage, warpage, or cracking. In the case of high-performance polymers, stress-relieving steps such as annealing can be used to eliminate these problems, since exposing the material to heat above its glass transition point allows the material to decompress back to a relaxed state.
Depending on the type of polymer used, the process parameters should be set to retain the tubing’s desired physical and mechanical properties. Nevertheless, because semicrystalline polymers have a narrow processing temperature range, the cooling parameters and cooling method used are critical for ensuring that crystallization does not occur in the tubing during the extrusion process.
Ballooning Materials Choices
In the 1980s, high- and low-density polyethylene materials were the most common polymers used in extrusion applications, remarks Tim Cortez, director of extrusion operations at Laguna Niguel, CA–based Interface Catheter Solutions. “But in the 1990s, the use of nylon and polyester evolved. Today, the most commonly used polymers are nylon 11, nylon 12, nylon-based copolymers such as Pebax, polyurethanes, PET, Hytrel, and engineered blends for targeted performance.” These materials are used to manufacture medical device components such as braided shafts, balloons, single- and multilumen tubes, and coextrusions. Extrusion shafts have evolved from simple single-lumen to complex multilayer and multilumen designs.
All of these materials have their pros and cons, Cortez notes. One drawback is that many of them contain gels, requiring that tubing manufacturers have proper understanding of the process technology and know how to either minimize or eliminate the gels in the tubing. Another drawback is residence time in the barrel. Because these materials have complex chemistries, they tend to be very sensitive to excess heat, shear, and process times. Because they degrade quickly, well-established processes, highly trained technicians, and well-maintained equipment are key to successfully extruding precision medical device tubing repeatably and reproducibly.
For example, improper resin feeding from the hopper to the screw—caused by the size or shape of the pellets—can affect the extrusion process significantly. “The standard geometry of pellets can be too large or their shape can be asymmetrical,” Cortez comments. “Sometimes, it’s necessary to use a grove throat or repelletize the resin to create micropellets, which tend to feed better.” However, while the repelletizing process can facilitate proper feeding, the extra heat cycle associated with the repelletizing process can cause the material to lose certain properties that are important for such components as balloons, Cortez adds. Thus, for balloon applications, manufacturers should avoid repelletizing or reprocessing the resin.
Some materials such as polyurethanes are not only heat- and moisture-sensitive but also time-sensitive. Often used in multilumen applications, polyurethanes must be processed quickly when they are used to produce tubing. “Time is your enemy because the material starts to degrade quickly,” Cortez warns. “Thus, you want to set it up efficiently using well-established processes and experienced extrusion technicians.”
In addition, getting a handle on the drying process is absolutely critical. “Extrusion shops should spend more time understanding the effects of drying polymers to achieve optimal performance,” according to Cortez. “The assumption that all nylon and polyurethane polymers dry in a set amount of time—say, two hours at 180ºF—is simply not true. You need to consider each polymer independently and determine its proper drying parameters.”
Despite these processing challenges, nylons and polyesters are not particularly temperature-sensitive and are easy to extrude, Cortez says. “For most polymers, you want to extrude the material through a ¾- to 1-in. extruder. While you need the right screw geometry to avoid feeding issues when using a ¾-in. extruder, most 1-in. extruders can process nylon and polyester materials without major challenges.”
Controlling the Extrusion Process
|To produce catheters with braid/coil reinforcement, VistaMed employs controlled extrusion inputs.|
The extrusion process is only as good as the manufacturer’s ability to control it. Thus, as minimally invasive techniques such as angioplasty drove the need for tubing with small diameters and thin walls, temperature control became critical for maintaining tight dimensional tolerances.
While in a molten state, a polymer's fluidity is relatively high, Tarmey explains. Under such conditions, a temperature change of only a few degrees can affect the process significantly. If the extrusion temperature is not high enough, the melted polymer will not be sufficiently homogeneous, causing some crystalline portions to remain solid. This unmelted crystalline phase can then initiate recrystallization during the cooling process, resulting in a heterogeneous morphology. Conversely, if the extrusion temperature is too high, the risk of material degradation increases, and it may be difficult to extrude the polymer because of its low viscosity.
Enhanced performance is achieved by incorporating a range of features into next-generation catheters. For example, braid/coil reinforcement provdes strength, rigidity, and torque control along the length of the catheter and also offers flexibility and kink resistance, enabling doctors to navigate tortuous body pathways. But to produce such features, controlled extrusion inputs are critical, Tarmey notes.
Ensuring tight tolerances depends on the equipment that extrusion providers use and how well they understand the engineering behind the equipment, its capabilities, and the tolerancing of the tooling, Cortez comments. “From a tolerance perspective, many extrusion shops won’t commit to ±0.0003 in. because they don’t have the right equipment technology to meet this tight tolerance range.”
To achieve such tight tolerances, gear-pump technology and properly engineered and understood downstream equipment is key. “For example, servo-driven pullers are a must, Cortez adds. “While all new pullers are equipped with servomotors, older equipment lacks this feature. And while some extrusion houses try to modify their old pullers, this is not an optimal solution.” Thus, investing in the latest equipment technology and training extrusion personnel is critical to making consistent, high-tolerance tubing for medical device applications.
Bob Michaels is senior technical editor at UBM Canon. Reach him at [email protected].
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